I. Field of the Invention
This invention relates to a process and apparatus for melting metals while reducing metal losses due to oxidation and/or recovering metal values from metal oxides already present, either as contaminants or as reinforcements or stabilizing particles in certain metal matrix composites and metal foams.
II. Discussion of the Prior Art
The recycling of metals and metal products is becoming increasingly important nowadays for economic and environmental reasons. In particular, so-called "light metals", such as aluminum and aluminum alloys, are recycled on a large scale. Used metals of this kind, e.g. from beverage cans, metal scrap and metal turnings, etc., can simply be remelted and cast into re-usable ingots or the like.
In the most common present practice, the metal is melted in a reverberatory furnace fired by a fuel such as natural gas. This is a very inefficient operation due to poor heat transfer between the hot combustion gases and the metal, causing a significant fraction of the input energy simply to be lost with the exhaust gases. The process also tends to convert a significant amount of the metal to oxide (dross) by hydrolysis during the melting operation and tends to contaminate the metal with hydrogen, which makes it necessary to de-gas the metal with a nitrogen/chlorine gas mixture prior to further use. This de-gassing process in turn tends to generate more dross contaminated with chlorides.
The generation of dross in these ways represents a loss of the metal from the melting operation and should thus be avoided, if possible. Moreover, the metal to be melted is usually already coated with or contaminated by a significant amount of oxide, particularly if it has undergone a preliminary decoating operation to remove organic contaminants by heating. Furthermore, certain metal products often contain large amounts of deliberately introduced oxide particles. For example, certain metal matrix composites use alumina or other oxides as reinforcement, and stabilized metal foams (for example as produced by a process described in U.S. Pat. No. 4,973,358) may contain alumina or other oxide particles as a stabilizing medium. Oxides incorporated into the metal in this way also represent a reduction of the total amount of potentially recoverable metal available during recycling.
When metals contain "reactive" (readily oxidizable) elements, such as aluminum, magnesium and lithium, either as majority or minority components, oxidation can be a particularly significant problem. These reactive elements oxidize quickly at elevated temperatures, such as those used for de-coating and melting, and form stable oxides which cannot easily be reduced by conventional techniques, such as carbothermal reduction. In the case of such metals and alloys, it is, therefore, particularly advantageous to avoid oxidation during melting operations.
In order to reduce such losses, it has been proposed that melting could be brought about by using electrical heating means and that the metal could be melted beneath an overlying layer of a molten salt mixture, e.g. a common chloride-based salt flux. The layer of molten salt would prevent exposure of the molten metal to atmospheric moisture and oxygen and would help to separate the metal, as it melts, from coating, contained or adhering oxides because the molten salt would preferentially wet the oxide. However, when this is attempted, the oxides introduced with the metal, or formed in reduced amounts during the melting operation, quickly accumulate in the molten salt layer until the salt layer becomes too viscous to be used further, whereupon it must be discarded. It is no longer environmentally acceptable to dispose of the resulting salt cake by simply dumping it in land-fill sites, so the consequent need for special disposal arrangements would increase the cost of operating such a process. Moreover, while the process would reduce the total amount of oxide formed during the melting operation, oxide present on or in the metal prior to the melting operation and the reduced amounts produced during the melting operation would still represent a significant loss of potentially recoverable metal.
In addition to conventional metals, metal matrix composite materials reinforced with metal oxides and metal foams stabilized by metal oxides, such as alumina, are being used in increasing amounts and are also candidates for recycling. As is the case for non-reinforced or stabilized materials, conventional methods do not lead to the complete recovery of the metal values in a convenient manner. There is, therefore, a need for an improved way of recycling these newer materials.
Metal losses could theoretically be avoided or reversed during a salt flux melting process if electrolysis of the oxide contained in the molten salt layer were to be carried out at the same time as, or subsequent to, the melting operation in order to reduce the oxide present in the salt layer to the elemental metal. Any metal oxide formed during such a melting process or introduced with the metal would then be converted to the metal by electrolysis, thus avoiding product losses. However, such an electrolysis process would not be easy to carry out on the scale required for commercial metal melting operations. A main problem is that current densities would have to be kept quite low in order to avoid decomposition of the chloride electrolyte (salt flux), since this would disadvantageously result in the generation of chlorine and/or the passivation of the anode surface (phenomena often referred to as "anode effects"). Even with very modest oxide contamination (of e.g. about 1% by weight) of the metal to be melted, very substantial quantities of oxide would be introduced into the metal melter at commercially acceptable melt rates (e.g. 1 to 20 tonnes per hour). If such quantities of oxide were to be completely eliminated by electrolysis, very high electrolysis currents would be required (e.g. about 100 kA for a 1% oxide-contaminated, 5 tonne/hour scrap metal stream), and this would greatly exceed the limiting current densities at which electrolyte decomposition would commence in metal melters of commercially feasible size.
Mutually related U.S. Pat. Nos. 4,758,316, 4,761,207 and 5,057,194 to Stewart et. al., all assigned to Aluminum Company of America, disclose melting processes carried out under molten chloride-containing salt layers while carrying out electrolysis to regenerate the metal from oxides. Two approaches are adopted. In a first of these approaches, the metal oxide collecting in the chloride-containing salt layer is itself converted to metal chloride by carbo-chlorination, and then the metal chloride is electrolyzed to regenerate the metal. In this case, the electrolysis of the chloride is not restricted to a limiting current density because the decomposition of the metal chloride and the electrolyte both result in chlorine generation without anode passivation, and chlorine generation is acceptable, in this process, especially since the chlorine can be used for the carbo-chlorination step. However, the process suffers from the significant disadvantages that both the gaseous reactants and the gaseous products are highly toxic and include chlorine, phosgene and carbon monoxide, and that the molten electrolyte must be transferred among a series of separate reaction vessels, which results in undue complexity and unacceptable plant size.
In a second approach, Stewart et. al. have suggested that the metal oxide in the molten salt layer may be electrolyzed directly, i.e. without prior conversion to a chloride, in a vessel separate from the main melting apparatus. It appears that Stewart has addressed the problem arising from the low limiting current density by increasing the effective surface area of the anode, e.g. by forming numerous holes or passages in the anode, in order to maintain low current densities while achieving high current flow. However, extending the surface areas of anodes in this way is not practical for processes using consumable anodes because suitably perforated electrodes are expensive to fabricate. Moreover, a relatively simple current distribution calculation demonstrates that only a small fraction of the total extended anode surface, i.e. that near to the external surface, would carry any significant amount of current. As a result, high current densities at the external surfaces of the anode are likely to result in the production of chlorine or passivation of the anode surface, while the interior of the anode would make little contribution to the electrolysis of the oxide.
Consequently, prior attempts to carry out electrolysis of oxide during metal melting operations have not been particularly practical and the concept has not been adopted for commercial scale operations.